Sport has no refuge from two possibly related human conditions: stress and sickness.

A little more than a year ago, Pep Guardiola left Barcelona, where he had built perhaps the most thrilling team in soccer outside of the Hungarian national team of the 1950s and the Brazil squad of 1970. He cited stress, the onus of responsibility for leaving a club that had won 14 of the 19 trophies it competed for in his four-year tenure as the coach.

On Friday, Tito Vilanova — Guardiola’s friend for more than 20 years, his assistant during that gluttony of achievement and his successor for one season — stepped down.

Vilanova has throat cancer.

He might have been able to handle the job as coach — the joy of pairing up Lionel Messi with Neymar and the task of trying to win the Champions League again. But as Vilanova wrote in a letter to the club and its fans over the weekend: “In the opinion of the doctors, and due to the treatment I have to follow, I can’t spend the time that a team like Barcelona would require as first coach to do my tasks. I’m calm, strong, and I will face this new stage in the process of my illness with full confidence that everything is going to be O.K.”

Everything was not O.K. One could see that from the ashen faces of the players Friday when they heard the news from the club’s president, Sandro Rosell, that their coach was leaving.

All the preseason planning and the conjecturing about new players yet to sign or teammates like Cesc Fàbregas possibly leaving became secondary to the realization that cancer strikes in every walk of life.

Vilanova is 44. He is a quiet man, private even in the glare of publicity that surrounds Barça. He has persevered from being a Barcelona youth team player who never made the first team to a coach who, as recently as Friday morning, was in his element watching and directing one of the world’s finest collections of players.

Photo

Pep Guardiola, Bayern Munich's coach. A little more than a year ago, Guardiola left Barcelona.Credit
Roland Weihrauch/European Pressphoto Agency

And they, so comfortable on the field and under the spotlight, didn’t know how to respond to this moment, either.

“Stay strong, Tito,” Messi posted on Facebook. “We are all with you in this fight!”

Vilanova wrote Saturday about “the human quality of this football team that can go through any obstacle” and of a club he hopes to continue to serve in another capacity. “Once I stop being the coach of the club,” he said, “I hope to have the tranquillity and privacy me and my family need right now.”

Tranquillity in the face of an illness that seemed to arrive almost as soon as he stepped into Guardiola’s shoes. Tranquillity when two operations and months of chemotherapy and radiation treatment in New York left him working by remote control, sometimes by video link across the Atlantic.

Barcelona and Vilanova held their season together to win back the Liga title from Real Madrid, equaling the record total of 100 points in the process. But with Vilanova so far from the training grounds, and with Messi and the captain Carles Puyol and others injured at crucial times, Barça was eclipsed, exposed by an aggregate score of 7-0 against Bayern Munich in the Champions League this spring.

The response to that devastation was to buy Neymar and to seek at least one top-notch defender. Barcelona has tried, so far unsuccessfully, to acquire Thiago Silva from Paris Saint-Germain and David Luiz from Chelsea.

How untimely the change of coach must be. Soccer is challenging at the best of times, and Rosell said on Spanish television Tuesday that the club was in the process of making the tough calls on which players to buy and which players to let go to continue its cycle of success.

Rosell had discussed the departures of striker David Villa and the playmaker Thiago Alcântara, the arrival of Neymar, and the efforts to sign defenders. He was asked about the coach. “Tito Vilanova,” he said, “he’s very eager and has a lot of energy.”

That would have been an intriguing night even without the weekend’s news. Bayern’s new coach is Guardiola, who is not on the best of terms with Rosell. As a result, the comradeship between Guardiola and Vilanova has not been as strong.

Earlier this month, Rosell told Spanish reporters that even though Vilanova spent two months having treatment in New York, where Guardiola was taking his sabbatical, the two men did not meet up there. Vilanova corrected that, saying they did meet, but only once, at the beginning of his treatment.

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“It wasn’t because I didn’t want to,” Vilanova added. “I was the one suffering, I was the one being left alone. In those moments, I needed him.”

All of this seems at odds with the friendship that prompted Guardiola to call Vilanova back to Barcelona as his assistant five years ago. And it runs counter to the admirable qualities of trust and sharing that have marked Guardiola’s career.

In the farewell letter he wrote Saturday, Vilanova thanked everyone at the club but did not mention Guardiola. At the same time, Guardiola was asked by journalists in Munich for a message to Vilanova.

“It’s hard to talk to a friend like Tito in German,” he said. “I love him so much. I wish the best for him and his family to tackle this stage with strength. This is very, very hard for me.”

The cancer, and the competitive strains of sports, are hard for everyone.

A version of this article appears in print on July 22, 2013, on Page D4 of the New York edition with the headline: Dealing With Cancer Amid Elite Soccer’s Pressure Cooker. Order Reprints|Today's Paper|Subscribe